1. Field of the Invention
The present invention relates to a power supply for a load control device, and more particularly, to a power supply for a load control device of a multiple location load control system, where the power supply draws a substantially symmetrical current, while powering an electrical load drawing an asymmetrical current.
2. Description of the Related Art
A conventional two-wire dimmer has two connections: a “hot” connection to an alternating-current (AC) power supply and a “dimmed hot” connection to the lighting load. Standard dimmers use one or more semiconductor switches, such as triacs or field effect transistors (FETs), to control the current delivered to the lighting load and thus to control the intensity of the light. The semiconductor switches are typically coupled between the hot and dimmed hot connections of the dimmer.
Smart wall-mounted dimmers may include a user interface typically having a plurality of buttons for receiving inputs from a user and a plurality of status indicators for providing feedback to the user. These smart dimmers typically include a microprocessor or other processing device for allowing an advanced set of control features and feedback options to the end user. An example of a smart dimmer is disclosed in commonly assigned U.S. Pat. No. 5,248,919, issued on Sep. 28, 1993, entitled LIGHTING CONTROL DEVICE, which is herein incorporated by reference in its entirety.
In order to provide a direct-current (DC) voltage VCC to power the microprocessor and other low-voltage circuitry, the smart dimmers typically include cat-ear power supplies. A cat-ear power supply draws current only near the zero-crossings of the AC source voltage and derives its name from the shape of the current waveform that it draws from the AC voltage source. Because the smart dimmer only has two terminals, the power supply must draw current through the connected lighting load. In order for the power supply to be able to draw sufficient current, the semiconductor switch must be non-conductive so that a sufficient voltage is available across the power supply. Thus, the semiconductor cannot be turned on for the entire length of a half-cycle, even when the maximum voltage across the lighting load is desired.
Sometimes, the power supplies of the smart dimmers are required to provide power to electrical loads that draw asymmetrical currents, for example, to an electrical load that draws a greater amount of current during the positive half-cycles than the negative half-cycles. In response to a load drawing an asymmetrical current, the prior art power supply causes a corresponding asymmetrical current to flow through the electrical load. If the electrical load is sensitive to asymmetrical currents, such as a magnetic low-voltage (MLV) lighting load, the lighting load may generate acoustic noise, which is undesirable. For example, acoustic noise may be generated if the current through the MLV lighting load has a DC component of approximately 0.3-0.4 A.
Accordingly, there is a need for a power supply for a load control device that is operable to draw a symmetrical current, while providing power to an electrical load that is drawing an asymmetrical current.